Trace metals such as Zn, Cd and Co are present at extremely low concentrations (nanomolar to picomolar) in surface seawater and they appear to be important in the primary production of the oceans. For the past several years, we have been studying the trace metal physiology of marine microalgae, both in the field and in the laboratory, using "clean" techniques to control the trace metal chemistry of the culture media. We are now beginning to understand the principal biochemical role of these elements in phytoplankton but it is becoming necessary to evaluate their intracellular speciation in vivo. XAS appears to be the most promising technique for studying in vivo the speciation of Zn, Cd and Co in cultures of representative marine microalgae. The application of XAS to field samples where the ambient population is dominated by one particular species is particularly interesting.